JP2012-031844A discloses a diesel engine which performs a main injection and a post injection. The post injection stimulates oxidation of soot generated in a combustion chamber and reduces emission of soot.
The diesel engine described in JP2012-031844A performs the post injection at such a timing that a portion of the fuel injected by a fuel injector reaches outside of a cavity formed in a piston. That is, the post injection is performed on expansion stroke when the piston descends from a top dead center of compression stroke. Therefore, the fuel of the post injection does not contribute much to generating an engine torque. Although the post injection is effective for reducing soot emission, it is disadvantageous for fuel efficiency.
In this regard, advancing the timing of the post injection is advantageous for increasing the engine torque and improving thermal efficiency. However, this causes an interval between the main injection and the post injection to be shorter; therefore, a fuel concentration becomes locally high and a new issue of soot generation arises.